The present invention generally relates to the bonding of two semiconductor wafers suitable for use in or the production of structures for micro-electronics, optics, or optronics applications. In particular, the invention relates to preparing an oxidized bonding surface of at least one of the wafers, wherein treatment parameters are chosen to provide etching that is sufficient to remove isolated particles from the oxidized surface but that is sufficiently weak to smooth the surface without creating rough patches thereon.
In order to ensure good contacting quality before bonding two wafers together, at least one and preferably both of the two surfaces to be bonded are cleaned to remove isolated particles or contaminants on wafer surfaces which are prejudicial to good bonding when located at the interface of two wafers. This is particularly the case when atomic species are implanted through an oxidized surface of a wafer to form a weakened area therein at a pre-set depth beneath the oxidized surface and to create a thin layer or film on the wafer surface. This thin layer is detached from the implanted wafer after it has been bonded to another substrate. An example of such a detachment process is the SMART-CUT® process, which is known to skilled person in the art (see “Silicon-on-Insulator Technology. Materials to VLSI”, 2nd edition, by Jean-Pierre Colinge, published by Kluwer Academic Publishers, pages 50 and 51), and which allows a film to be removed from a wafer for transfer to another wafer. A semiconductor-on-insulator structure such as an SOI (Silicon On insulator) structure can be made in this manner by transferring a thin silicon film from a donor wafer to a receiver wafer.
With the increase of miniaturization of electronic components formed in semiconductor layers, manufacturers of semiconductor-on-insulator substrates are increasingly asked to make semiconductor-on-insulator structures that include thinner and thinner semiconductor films. Thus, it is vitally important to improve the quality of a transferred layer and therefore to improve removal techniques. Consequently, the quality of the bond between the layer to be transferred and the receiver substrate is essential in order to ensure good removal, wherein the quality of the bond is mainly measured by the bonding energy between the two wafers. To ensure that the contact area of the two wafers to be joined is of good quality, it is necessary to implement a cleaning operation to clean at least one of the two bonding surfaces.
Cleaning is conducted to remove those particles which are enclosed in the bonding interface and which can lead to superficial blisters being formed in the structure obtained after detachment or to portions of the wafer that were not transferred between the zone at which the species were implanted and the surface of the structure. Such blisters can multiply or grow during a heat treatment, such as that conducted during or after bonding to strengthen the bond. Thus, it is important to find a means for removing such particles and contaminants from bonding surfaces.
A trend in the prior art is to chemically treat the wafers in stages prior to bonding. To clean the surface of a wafer of oxidized or non-oxidized semi-conductor material, the known technique is to use a treatment called RCA. The RCA treatment includes a first bath of Standard Clean 1(SC1) solution that includes ammonium hydroxide (NH4OH), hydrogen peroxide (H2O2) and deionised water, generally at a temperature of between 30° C. and 80° C., and then a second bath of Standard Clean 2 (SC2) solution, which contains hydrochloric acid (HCl), and hydrogen peroxide (H2O2), and deionised water, generally at a temperature of between 70° C. and 90° C. The first bath is intended mainly for removing isolated particles on the wafer surface and for removing particles buried in the vicinity of the surface and to prevent them from resettling. Further, that first SC1 dip can improve the hydrophilicity of the surface of the wafer to be bonded, and is used because the hydrophilic property of the surface constitutes an essential condition for good bonding between the wafers. The SC2 solution mainly removes any metallic contamination that has settled on the wafer surface, in particular, by forming chlorides.
The metal removal efficiency of the SC2 solution is typically between 95% and 99% at temperatures between ambient and 90° C. The depth of chemical action into the wafer is typically from about 1 angströms (Å) to about 10 Å at these temperatures. For this reason, the SC2 treatment is usually employed at temperatures in the range 70° C. to 90° C.
Even after implementing such chemical treatments, however, the resulting surfaces have roughness or rough patches, which can, in some cases, be more significant than that existing prior to treatment due to the attack of the surface by the cleaning solutions. Such roughness or rough patches on the surface of the wafers alter the bonding energy of the wafers because they have a relatively high Root Mean Square (RMS) value, measured in angströms. The presence of isolated particles or contaminants on the surface of the wafers can also be detrimental to good bonding of the wafers when these contaminants are found at its interface. After bonding, these particles which are enclosed at the bonding interface, may cause surface blisters to form in the structure obtained after using a SMART-CUT® detachment technique, and/or cause surface blisters in areas not transferred between the area at the level of which the species were implanted and the surface of the structure. These blisters increase in size and/or grow during any subsequent heat treatment, for example, a heat treatment used after bonding to strengthen the bond.
One known solution for increasing separation of the isolated particles is to conduct the chemical treatment while applying ultrasonic waves. These waves cause the isolated particles to vibrate and therefore to separate off. It is preferable, however, to avoid implementing an additional process when cleaning the wafers to avoid complicating the cleaning stage. Furthermore, additional equipment would be required in order to generate the ultrasonic waves.
European patent application 0 971 396 suggests a cleaning technique consisting of preparing a such implanted surface by implementing three cleaning steps, an SC1 treatment, followed by an SC2 treatment, and finally followed by another SC1 treatment. The three steps of this process have a total duration of SC1 treatment of about 8 minutes (2×4 minutes) at 80° C., and of SC2 treatment of about 4 minutes at 80° C. This does not resolve the problems of the art and thus there still remains a need for techniques that provide improvements in the bond quality between joined wafers. It is therefore necessary to carry out an RCA treatment that is adapted to optimize both cleaning and bonding actions. Such a treatment is now provided by the present invention.